Various organic solvents have been used as cleaning liquids for the removal of contaminants from contaminated articles and materials. Certain fluorine-containing organic compounds such as 1,1,2-trichloro-1,2,2-trifluoroethane have been reported as useful for this purpose, particularly with regard to cleaning organic polymers and plastics which may be sensitive to other more common and more powerful solvents such as trichloroethylene or perchloroethylene. Recently, however, there have been efforts to reduce the use of certain compounds such as trichlorotrifluoroethane which also contain chlorine because of a concern over their potential to deplete ozone, and to thereby affect the layer of ozone that is considered important in protecting the earth's surface from ultraviolet radiation.
Boiling point, flammability and solvent power can often be adjusted by preparing mixtures of solvents. For example, certain mixtures of 1,1,2, -trichloro-1,2,2-trifluoroethane with other solvents (e.g., isopropanol and nitromethane) have been reported as useful in removing contaminants which are not removed by 1,1,2-trichloro-1,2,2-trichloroethane alone, and in cleaning articles such as electronic circuit boards where the requirements for a cleaning solvent are relatively stringent, i.e., it is generally desirable in circuit board cleaning to use solvents which have low boiling points, are non-flammable, have low toxicity, and have high solvent power so that flux such as rosin and flux residues which result from soldering electronic components to the circuit board can be removed without damage to the circuit board substrate).
While boiling point, flammability, and solvent power can often be adjusted by preparing mixtures of solvents, the utility of the resulting mixtures can be limited for certain applications because the mixtures fractionate to an undesirable degree during use. Mixtures can also fractionate during recovery, making it more difficult to recover a solvent mixture with the original composition. Azeotropic compositions, with their constant boiling and constant composition characteristics, are thus considered particularly useful.
Azeotropic compositions exhibit either a maximum or minimum boiling point and do not fractionate upon boiling. These characteristics are also important in the use of the solvent compositions in certain cleaning operations, such as removing solder fluxes and flux residues from printed circuit boards. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if the mixtures were not azeotropes, or azeotrope-like, would result in mixtures with changed compositions which may have less desirable properties (e.g., lower solvency for contaminants such as rosin fluxes and/or less inertness toward the substrates such as electrical components).
Azeotropic characteristics are also desirable in vapor degreasing operations where redistilled material is usually used for final rinse-cleaning. Thus, the vapor defluxing or degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point (i.e., is an azeotrope or is azeotrope-like) fractionation will occur and undesirable solvent distribution may act to upset the safety and effectiveness of the cleaning operation.
A number of azeotropic compositions based upon halohydrocarbons containing fluorine have been discovered and in some cases used as solvents for the removal of solder fluxes and flux residues from printed circuit boards and for miscellaneous vapor degreasing applications. For example, U.S. Pat. No. 2,999,815 discloses the azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with acetone; U.S. Pat. No. 3,903,009 discloses a ternary azeotrope of 1,1,2-trichloro-1,2,2-triflouroethane with nitromethane and ethanol; U.S. Pat. No. 3,573,213 discloses an azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane; U.S. Pat. No. 3,789,006 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane and isopropanol; U.S. Pat. No. 3,728,268 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with acetone and ethanol; U.S. Pat. No. 2,999,817 discloses the binary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and methylene chloride (i.e., dichloromethane); and U.S. Pat. No. 4,715,900 discloses ternary compositions of trichlorotrifluoroethane, dichlorodifluoroethane, and ethanol or methanol.
As noted above, many solvent compositions which have proven useful for cleaning contain at least one component which is a halogen-substituted hydrocarbon containing chlorine, and there have been concerns raised over the ozone depletion potential of halogen-substituted hydrocarbons which contain chlorine. Efforts are being made to develop compositions which may at least partially replace the chlorine containing components with other components having lower potential for ozone depletion. Azeotropic compositions of this type are of particular interest.
Means of synthesizing various fluorine-substituted alkanes have been reported.
U.S. Pat. No. 2,917,559 discloses a vapor phase process for the production of 2-fluoropropane by the reaction of HF and propylene over an activated carbon catalyst.
U.S. Pat. No. 2,975,220 discloses compounds of the general formula R(CH.sub.2 CF.sub.2)nQ, where n is an integer and Q is halogen or hydrogen and R is a halogenated radical. These compounds (e.g., CF.sub.3 CH.sub.2 CF.sub.2 CF.sub.2 CF.sub.3) may be prepared by reacting vinylidene fluoride with certain telogens.
U.S. Pat. No. 3,520,786 discloses a process for the preparation of cycloalkanes by electrolyzing a solution of halocarbons having 3-6 ring carbons of the general composition C(R.sub.1)RR--C(.sub.1-4)RR--C(R.sub.2)RR, where R may be an alkyl group, hydrogen, or a halogen; R.sub.1 is halogen; and R.sub.2 may be a halogen, quarternary ammonium salt or a tosylate; and isolating the corresponding cycloalkane.
U.S. Pat. No. 4,902,838 discloses a process for the isomerization of C.sub.2 to C.sub.6 hydrofluorocarbons having lesser thermodynamic stability to hydrofluorocarbons having greater thermodynamic stability by isomerization in the vapor phase of at least one C.sub.2 to C.sub.6 saturated hydrofluorocarbon with a catalyst comprising aluminum fluoride. The isomerization of 1,1,2,2-tetrafluoroethane, a vicinal-dihydro fluorocarbon, to 1,1,1,2-tetrafluoroethane, a geminal-dihydro fluorocarbon, is exemplified.
Eur. Pat. Appln. No. 365,296 discloses a process for the preparation of 1,1,1,2-tetrafluoroethane by the isomerization of 1,1,2,2-tetrafluoroethane over a fluorination catalyst. The only catalyst examplified is chromia.
C. Zhanxun et al., Proc. Annu. Int. Conf. Plasma Chem. Technol., 4th, Meeting Date 1987, 173-9 (1989) and C. Zhanxun et al., Adv. Low-Temp. Plasma Chem., Technol., Appl., 2, 265-73 (1988) discloses the formula CF.sub.3 CF.sub.2 CH.sub.2 CF.sub.2 CF.sub.3 as a theoretical product from the degradation of plasma-polymerized tetrafluoroethylene.
There are also means of synthesizing various fluorine-substituted alkenes. For example, U.S. Pat. Nos. 4,820,883 and 4,820,884 disclose the use of activated carbon for the preparation of unsaturated fluorocarbons by defluorinating perfluoro compounds.